1. Technical Field
The present invention relates to a glossing device and an image forming apparatus incorporating the same, and more particularly, to a glossing device that processes a toner image with heat and pressure on a recording medium, and an electrophotographic image forming apparatus, such as a photocopier, facsimile machine, printer, plotter, or multifunctional machine incorporating several of these features, which incorporates such a glossing capability.
2. Background Art
In electrophotographic image forming apparatuses, such as photocopiers, facsimile machines, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the imaging process may be followed by a fixing process using a fixing device, which permanently fixes the toner image in place on the recording medium by melting and setting the toner with heat and pressure.
FIG. 1 is an end-on, axial view of an example of a fixing device 300 used in electrophotographic image formation.
As shown in FIG. 1, the fixing device 300 includes a dual-roller assembly formed of a pair of opposed rotary members disposed parallel to each other, one being a fuser roller 301 internally heated with a heat source 302 and rotatably driven with a rotary motor, and the other being a pressure roller 302 elastically biased against the fuser roller 301 to form a fixing nip N therebetween.
During operation, the heat source 302 imparts heat from inside the fuser roller 301 to maintain the outer surface of the roller 301 at an operational temperature greater than a softening temperature of toner in use. Then, as the fuser roller 301 rotates in a given rotational direction (clockwise in the drawing) to in turn cause the pressure roller 302 in an opposite rotational direction (counterclockwise in the drawing), a recording sheet S enters the fixing nip N with its printed surface (i.e., the side on which a toner image T is deposited) facing the fuser roller 301, which melts and fuses toner with heat from the fuser roller 301 and pressure between the opposed rollers 301 and 303. After passage through the fixing nip N, the recording sheet S cools to cause the molten toner thereon to cool and solidify, resulting in the toner image T fixed in place on the recording sheet S.
One important factor that determines performance of fixing process is its capability to impart high, uniform gloss to the resulting print, which is particularly required by modern image forming apparatuses which accommodate a wide range of printing applications with different levels of image quality, using various types of recording media. In particular, printing of photographs and computer-generated images, which typically contain a wide, complete range of visible colors, necessitates a higher level of image quality and uniformity of image gloss than is required for conventional monochrome image formation.
Various techniques have been proposed to meet a growing demand for printers with high-gloss, high-quality imaging performance. Some such techniques employ a special, transparent toner, called “clear toner”, for creating a transparent glossy effect on those areas of a recording medium where no color toner is deposited; others address duplex printing with a uniform, glossy finish on both sides of a recording medium. Among these, several techniques are directed to development of a more sophisticated fixing process.
Structurally, a fixing device with a glossing capability may be constructed of an endless rotary belt on which a recording medium is conveyed while subjected to heat and pressure. The endless belt is looped for rotation around multiple parallel rollers, including a heated roller and a separator roller, with a pressure roller disposed opposite the heated roller via the belt to form a fixing nip therebetween. During operation, a recording medium is conveyed through the fixing nip to process a toner image under heat and pressure. After passage through the fixing nip, the recording medium closely contacts the belt as the belt moves from the heated roller toward the separator roller, and separates from the belt as the belt passes around the separator roller.
For example, a fixing system has been proposed which includes a primary fixing unit and a secondary fixing unit disposed downstream from the primary fixing unit. The secondary fixing unit includes a pair of opposed belt assemblies, each of which consists of an endless rotary belt entrained around a heat roller and a stripper roller. The two heat rollers and the two stripper rollers press against their respective counterparts on the opposite side of the glossing device to define an elongated area of contact or nip extending from between the heat rollers to between the stripper rollers, along which a recording medium is conveyed between the rotating belts.
In this fixing system, after initially passing through the primary fixing unit to fix a toner image in place with heat and pressure, a recording medium is conveyed to the secondary fixing unit. Upon entry into the secondary fixing unit, the recording medium is heated between the heat rollers to re-melt the once-fixed toner image thereon, and then is gradually cooled as it passes toward the stripper rollers along the length of the elongated nip, causing the toner image to conform to the smooth surface of the endless belt. As the recording medium exits the elongated nip, the toner image solidifies and thus exhibits increased gloss and smooth appearance.
Also, another belt-based fixing system has been proposed which includes a thermal pre-fixing unit and a gloss adjustment unit. The pre-fixing unit consists of a pair of opposed heated rollers pressing against each other to form a pre-fixing nip therebetween. The gloss adjustment unit consists of a smooth, endless rotary belt entrained around a pair of motor-driven and idler rollers, with a pressure roller opposite the motor-driven roller to form a main, fixing nip therebetween.
In this fixing system, a recording medium is initially passed through the pre-fixing unit, which renders an unfixed powder toner image into a semi-fluid, soft pliable state. After pre-fixing, the recording medium is conveyed to the gloss adjustment unit with the toner image pressed against the endless belt, which imparts gloss to the toner image as the molten toner gradually cools and solidifies while conforming to the smooth surface of the belt. The gloss adjustment unit adjusts glossiness of the toner image by adjusting a distance or duration during which the toner image travels on the belt downstream from the fixing nip.
To date, belt-based glossing devices are designed with a belt cooler for cooling an endless rotary belt during conveyance of a recording medium downstream from a fixing nip, so as to provide uniform cooling and proper separation of the recording medium from the belt after fixing and glossing a toner image thereon.
For example, the aforementioned fixing system with the gloss adjustment capability is equipped with a cooling device, such as a heat-dissipating fin or an electrical fan, disposed inside the loop of the endless belt. This cooling device serves to cool the belt opposite where the belt faces the recording medium conveyed downstream from the fixing nip, which in turn cools the recording medium as well as a toner image printed thereon.
Although generally successful for their intended purposes, conventional techniques for cooling an endless rotary belt in a glossing device have several drawbacks.
One drawback is that the belt cooler often fails to accommodate different thermal properties of the endless belt which depend on specific material and application of the glossing process.
For example, a relatively high heat capacity of the belt material reduces thermal efficiency of the belt cooler, which can lead to variations in glossing performance where the belt accumulates substantial amounts of heat during sequential processing of a large number of recording media, which causes the resulting prints to exhibit varying levels of image gloss over time. On the other hand, a relatively low heat capacity of the belt material makes it difficult to sufficiently cool the toner image, particularly where the recording medium in use is a thick substrate of paper that exhibits a greater heat capacity than that of the endless belt. Increasing the length of belt to remedy the problem does not always work, since it would cause a concomitant increase in overall size of the equipment which is not desirable for application to today's compact printers.
Another drawback arises where the glossing device is applied to an image forming apparatus that incorporates a duplex printing capability to allow image formation on a pair of opposed, first and second surfaces of a recording medium. During duplex printing where the glossing device processes a duplex-printed recording medium that has a first toner image already fixed and glossed on its first surface, and a second toner image fixed and yet to be glossed on its second surface, applying heat and pressure to the recording medium for glossing the second toner image can affect the gloss and quality of the first toner image, as the first toner image remains substantially hot and thus is readily deformed under pressure as the recording medium passes through the glossing nip.